1. Field of the Invention
The invention relates to pipeline laying and more particularly to the laying of a pipeline on the sea bed.
2. Brief Review of the Prior Art
It has been proposed previously to lay a pipeline on the sea bed either by unwinding the pipeline from a reel carried on a vessel or by welding lengths of pipe on the vessel to form the pipeline and then laying the pipeline on the sea bed. The use of a reel requires a comparatively low tension in the pipeline so reducing the thrust requirements of the vessel. This allows the pipeline to be laid in very deep water. This method is, however, limited by the capacity of the reel, which may, for example, be 20 km of 305 mm (12 inch) pipe. In order to obtain further pipeline for laying, the vessel must travel to one of the scarce dedicated bases for the fabrication of pipeline, moor and take onto the reel a fresh length of pipeline.
The welding of a pipeline from lengths of pipe has been achieved in two different ways. In one way, the pipeline is fabricated on the vessel and then bent elastically over a stern structure commonly known as a stinger before being allowed to assume the natural catenary between the vessel and the sea bed. This requires very high tensions to be maintained in the pipeline and consequently very high thrust from the vessel, in order to avoid buckling of the pipeline at the end of the stinger as it is laid overboard. Even the most modern of vessels can only lay pipeline to a depth of 600 metres by this method but it has the advantage that the vessel can be re-supplied with pipe lengths while laying the pipeline so that the length of pipeline that can be laid in a stretch is almost limitless.
An alternative is to prefabricate a number of pipe lengths (say three or more) into a pipe stalk which is then loaded into a generally horizontal ramp extending along the vessel and pivoted about the stern of the vessel. By tilting the ramp the pipe stalk may then be aligned with a separate ramp, also pivoted about the stern, which is adjustably fixed at the angle of the top end of the pipe catenary. The pipe stalk is then welded to the pipeline and passed over the stern. In this method there is no bending of the pipe either at the top end of the catenary, or on board the vessel, but the ramp must be more than 150 m long in order to accommodate sufficiently long pipe stalks to achieve sensible lay rates.